Mean Sea Level Datums (Normal-Orthometric Heights)

Thirteen primary mean sea level (MSL) vertical datums are used in New Zealand (shown in the image below).

These vertical datums have been established by determining MSL at a tide-gauge and then transferring this level by precise levelling to benchmarks in the surrounding hinterland. To account for changes in the Earth's gravity field an approximate orthometric correction is also applied to the levelling observations to give normal-orthometric heights in relation to MSL at the tide-gauge origin of that datum.

Sea level is known to vary around the coast of New Zealand. This means that the level of MSL determined each datum's tide-gauge will be different and that offsets will occur between adjacent datums. It is therefore important that the vertical datum of a height is checked before it is used. Also, in most cases the level of MSL for the vertical datums was determined over 50 years ago and has not been updated since then (due to the effect of sea level rise for instance).

Normal-orthometric heights can be related to the NZGD2000 by using a geoid model (eg, NZGeoid09) and the appropriate datum offset. The conversion of heights between different systems is described on the height conversion pages.

The accuracy of normal-orthometric heights is classified by a series of Orders. The Order of a height is related to the procedure used to observe it. This Order classification is different to that used for ellipsoidal heights.